It's not really up to a student "informer" to prove anything. All you can (or should) do if you're reporting possible plagiarism, is describe what you have seen that you think the instructor hasn't, in a non-judgmental and objective way.
– ff524♦Sep 1 '14 at 22:04

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@ff524: I don't know about that, I would be more inclined to keep my mouth shut if I wasn't too sure something is plagiarism. At the very least you risk your reliability as a signal source (aka if you're wrong then people are less likely to care about your thoughts later), and in the worst case the other person is actually innocent and finds out and becomes offended, ruining your relationship in the process.
– MehrdadSep 1 '14 at 22:25

@Mehrdad - Yes, but in maths' language, that's the ''trivial solution''. You are right, when OP isn't sure, he certainly should keep quiet. But when he uses the words ''academic dishonesty'' in an in-principle type of a question, isn't it taken for granted that he has reasons to be sure? Then, he's looking for ''non-trivial solutions''.
– 299792458Sep 2 '14 at 7:01

@ff524 - But if you don't put the requirement of proving the claims, won't there be too many false, or misguided claims? And all that whoever-is=the-judge will do in a day's work is to attend to frivolous dishonesty complaints.
– 299792458Sep 2 '14 at 7:06

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@ff524 - Because everyone won't act in good faith. What if the claims are used to extract ''revenge'' for a bad viva-voce? You are right, the admins are better placed to judge, but they can certainly reduce their workload if they start asking for some evidence. Or at least, a ''how do you know ?'' Otherwise, this is all that they will do in a day's work.
– 299792458Sep 2 '14 at 7:31